Asymmetric Polytetrafluoroethylene Composite Having A Macro-Textured Surface And Method For Making The Same

ABSTRACT

Polytetrafluoroethylene (PTFE) composite articles that have a macro textured surface. The composite articles include at least two different PTFE membranes in a layered or stacked configuration. The composite article has a macro textured surface characterized by a plurality of strands raised from the surface of the PTFE membrane. The strands may be formed of either interconnected nodes of PTFE or of at least one nodal mass of PTFE and have a length equal to or greater than about 1.5 mm. The macro textured surface provides a topography to the first PTFE membrane. The composite articles have a bubble point from about 3.0 psi to about 200 psi, a thickness from about 0.01 to about 3.0 mm, and a bulk density from about 0.01 g/cm3 to about 1.0 g/cm3.

FIELD

The present invention relates generally to polytetrafluoroethylene(PTFE) membranes, and more specifically, to polytetrafluoroethylenecomposite articles having a macro textured surface. Methods for makingthe composite articles are also disclosed.

BACKGROUND

A conventional method of manufacturing expanded PTFE (ePTFE) layer isdescribed in U.S. Pat. No. 3,953,566 to Gore. In the methods describedtherein, a PTFE paste is formed by combining a PTFE resin and alubricant. The PTFE paste may be extruded. After the lubricant isremoved from the extruded paste, the PTFE article is stretched to createa porous, high strength PTFE article. The expanded PTFE layer ischaracterized by a porous, open microstructure that has nodesinterconnected by fibrils.

ePTFE articles with a variety of microstructures of nodes and fibrilsare known in the art. Some such articles are described in U.S. Pat. Nos.4,902,423, 5,814,405, 5,476,589, 6,342,294.

U.S. Pat. No. 6,780,497 describes a method using laser to surface modifyan ePTFE structure to create a macro-roughened surface that has thecapability to remain microporous throughout. The process creates astructure including gnarled nodes.

It is desirable to modify the ePTFE surface without using such anextraneous process and still be able to create a patterned ePTFEsurface.

SUMMARY

One embodiment of the invention relates to a composite article thatincludes (1) a first polytetrafluoroethylene (PTFE) membrane that has afirst surface and a second surface and (2) a second PTFE membranepositioned on the second surface of the first membrane where the firstsurface of the first PTFE membrane includes a plurality of strandsraised from the first surface. At least one of the strands has a lengthgreater than about 1.5 mm. The strands include interconnected nodes ofPTFE. The composite article has a bulk density from about 0.01 g/cm³ toabout 1.0 g/cm³, a bubble point from about 3.0 psi to about 200 psi, anda thickness from about 0.01 mm to about 3.0 mm. In at least oneembodiment, the strands are non-linear and form a visible pattern on thefirst surface of the first PTFE membrane. In at least one exemplaryembodiment, the second PTFE membrane has a matrix tensile strength thatis at least 1.5 times greater than the matrix tensile strength of thefirst PTFE membrane in both the x- and the y-directions. The compositearticles have a macro textured surface which can be optically observedand which provides a topography to the first PTFE membrane.

A second embodiment of the invention relates to a composite article thatincludes (1) a first polytetrafluoroethylene (PTFE) membrane that has afirst surface and a second surface and (2) a second PTFE membranepositioned on the second surface of the first membrane where the firstsurface of the first PTFE membrane includes a plurality of strandsraised from the first surface. At least one of the strands has a lengthgreater than about 1.5 mm. The strands include at least one nodal massof PTFE. The composite article has a bulk density from about 0.01 g/cm³to about 1.0 g/cm³, a bubble point from about 3.0 psi to about 200 psiand a thickness from about 0.01 mm to about 3.0 mm. In at least oneembodiment, the strands are non-linear and form a visible pattern on thefirst surface of the first PTFE membrane. In at least one exemplaryembodiment, the second PTFE membrane has a matrix tensile strength thatis at least 1.5 times greater than the matrix tensile strength of thefirst PTFE membrane in both the x- and the y-directions. The compositearticles have a macro textured surface which can be optically observedand which provides a topography to the first PTFE membrane.

A third embodiment of the invention relates to a composite articleproduced by the process including (1) obtaining a first PTFE membranehaving a first surface and a second surface, (2) obtaining a second PTFEmembrane having a matrix tensile strength that is greater than a matrixtensile strength of the first PTFE membrane, (3) positioning the secondPTFE membrane on the second surface of the first membrane to form alayered product, (4) expanding the layered product in the y direction atan engineering strain rate from about 0.5%/sec to about 300%/sec and astretch rate from about 10% to about 350% to form an expanded product,and (4) expanding the expanded product in the x-direction at anengineering strain rate from about 3.0% to about 600% and a stretchratio from about 0% to about 2000% to form a composite article. Thefirst surface of the composite article includes a plurality of strandsraised from the first surface. At least one of the strands has a lengthequal to or greater than about 1.5 mm. The first surface of thecomposite article corresponds to the first surface of the first PTFEmembrane. In a further step, the composite article may be sintered. Anadhesive may be applied to one of the first PTFE membrane and the secondPTFE membrane to adhere the first PTFE membrane to the second PTFEmembrane.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the disclosure and are incorporated in and constitute apart of this specification, illustrate embodiments, and together withthe description serve to explain the principles of the disclosure.

FIG. 1 is an optical microscopic image of the surface of the first PTFEmembrane in the composite article of Example 1 taken at 7× in accordancewith one embodiment of the invention;

FIG. 2 is an optical microscopic image of the surface of the first PTFEmembrane in the composite article of Example 1 taken at 32× according toan embodiment of the invention;

FIG. 2A is a scanning electron micrograph of the surface of the firstPTFE membrane in the composite article of Example 1 taken at 100×according to at least one embodiment of the invention;

FIG. 3 is an optical microscopic image of the surface of the first PTFEmembrane in the composite article of Example 2 taken at 7× in accordancewith an embodiment of the invention;

FIG. 4 is an optical microscopic image of the surface of the first PTFEmembrane in the composite article of Example 2 taken at 32× inaccordance with an embodiment of the invention;

FIG. 4A is a scanning electron micrograph of the surface of the firstPTFE membrane in the composite article of Example 2 taken at 100×according to at least one embodiment of the invention;

FIG. 5 is an optical microscopic image of the surface of the first PTFEmembrane in the composite article of Example 3 taken at 7× in accordancewith an embodiment of the invention;

FIG. 6 is an optical microscopic image of the surface of the first PTFEmembrane in the composite article of Example 3 taken at 32× inaccordance with an embodiment of the invention;

FIG. 6A is a scanning electron micrograph of the surface of the firstPTFE membrane in the composite article of Example 3 taken at 100×according to at least one embodiment of the invention;

FIG. 7 is an optical microscopic image of the surface of the first PTFEmembrane in the composite article of Example 4 taken at 7× in accordancewith an embodiment of the invention;

FIG. 8 is an optical microscopic image of the surface of the first PTFEmembrane in the composite article of Example 4 taken at 32× inaccordance with an embodiment of the invention;

FIG. 8A is a scanning electron micrograph of the surface of the firstPTFE membrane in the composite article of Example 4 taken at 100×according to at least one embodiment of the invention;

FIG. 9 is an optical microscopic image of the surface of the first PTFEmembrane in the composite article of Example 5 taken at 7× in accordancewith an embodiment of the invention;

FIG. 10 is an optical microscopic image of the surface of the first PTFEmembrane in the composite article of Example 5 taken at 32× inaccordance with an embodiment of the invention;

FIG. 10A is a scanning electron micrograph of the surface of the firstPTFE membrane in the composite article of Example 5 taken at 100×according to at least one embodiment of the invention;

FIG. 11 is an optical microscopic image of the surface of the first PTFEmembrane in the composite article of Example 6 taken at 7× in accordancewith an embodiment of the invention;

FIG. 12 is an optical microscopic image of the surface of the first PTFEmembrane in the composite article of Example 6 taken at 32× inaccordance with an embodiment of the invention;

FIG. 12A is a scanning electron micrograph of the surface of the firstPTFE membrane in the composite article of Example 6 taken at 100×according to at least one embodiment of the invention;

FIG. 13 is an optical microscopic image of the surface of the first PTFEmembrane in the composite article of Example 7 taken at 7× in accordancewith an embodiment of the invention;

FIG. 14 is an optical microscopic image of the surface of the first PTFEmembrane in the composite article of Example 7 taken at 32× inaccordance with an embodiment of the invention;

FIG. 14A is a scanning electron micrograph of the surface of the firstPTFE membrane in the composite article of Example 7 taken at 100×according to at least one embodiment of the invention;

FIG. 15 is an optical microscopic image of the surface of the first PTFEmembrane in the composite article of Example 8 taken at 7× in accordancewith an embodiment of the invention;

FIG. 16 is an optical microscopic image of the surface of the first PTFEmembrane in the composite article of Example 8 taken at 32× inaccordance with an embodiment of the invention;

FIG. 16A is a scanning electron micrograph of the surface of the firstPTFE membrane in the composite article of Example 8 taken at 100×according to at least one embodiment of the invention; and

FIG. 16B is a scanning electron micrograph of the surface of the firstPTFE membrane in the composite article of Example 8 taken at 2500×according to at least one embodiment of the invention.

GLOSSARY

As used herein, the term “optically observed” is meant to denote that anobject can be observed with the use of an optical microscope and/or withthe naked eye.

As used herein, the term “on” is meant to denote an element, such as apolytetrafluoroethylene (PTFE) membrane, is directly on another elementor intervening elements may also be present.

As used herein, the term “biaxial” is meant to describe a polymer,membrane, preform, or article that is expanded in at least twodirections, either simultaneously or sequentially.

As used herein, the term “lubricant” is meant to describe a processingaid that includes, and in some embodiments, consists of, anincompressible fluid that is not a solvent for the polymer at processingconditions. The fluid-polymer surface interactions are such that it ispossible to create a homogenous mixture.

As used herein, the term “wet state” is meant to describe a PTFEmembrane that has not been dried to remove lubricant.

The term “dry state” as used herein is meant to describe a PTFE membranethat has been dried to remove lubricant.

“Fine powder PTFE” as used herein is meant to denote that the PTFE resinwas prepared by an aqueous dispersion polymerization technique.

As used herein, the terms “x-direction” and “y-direction” are meant todenote transverse and longitudinal directions, respectively.

DETAILED DESCRIPTION

Persons skilled in the art will readily appreciate that various aspectsof the present disclosure can be realized by any number of methods andapparatus configured to perform the intended functions. It should alsobe noted that the accompanying drawing figures referred to herein arenot necessarily drawn to scale, but may be exaggerated to illustratevarious aspects of the present disclosure, and in that regard, thedrawing figures should not be construed as limiting.

The present invention is directed to polytetrafluoroethylene (PTFE)composite articles that have a macro textured surface which can beoptically observed. The composite articles include at least twodifferent PTFE membranes in a layered or stacked configuration, eachPTFE membrane having a microstructure of nodes interconnected byfibrils. A first PTFE membrane in the composite article has amicrostructure that is more “open” than a second PTFE membrane in thecomposite article. The composite articles have a bubble point from about3.0 psi to about 200 psi, a thickness from about 0.01 to about 3.0 mm,and a bulk density less than about 1.0 g/cm³.

As discussed above, the composite article is formed of at least two PTFEmembranes, which may be formed from the same or different PTFE startingmaterial. In addition, the first and second PTFE membranesmicrostructures that differ from each other. The difference between themicrostructure of the first PTFE membrane and the microstructure of thesecond PTFE membrane, and thus the asymmetry of the composite article,may be caused by, for example, a difference in pore size, a differencein node and/or fibril geometry or size, and/or a difference in density.Notwithstanding the mechanism utilized to achieve differentmicrostructures achieved within the composite article, the first PTFEmembrane possesses a microstructure that is more “open” than themicrostructure of the second PTFE layer. As used herein, the term “open”as opposed to “tight” means that the pore size of the first “open”microstructure is larger than that of the second “tight” microstructureas evidenced by bubble point or any suitable method for characterizingpore size.

The PTFE starting material used for forming the first and second PTFEmembranes can be of any type of PTFE resin which lends itself to theformation of fibrils and nodes upon expansion. In one exemplaryembodiment, the PTFE starting material may be a PTFE homopolymer or ablend of PTFE homopolymers. In another embodiment, the PTFE startingmaterial may be a blend of a PTFE homopolymer and a PTFE copolymer inwhich comonomer units are not present in amounts which cause thecopolymer to lose the non-melt processible characteristics of a purehomopolymer PTFE. Examples of suitable comonomers in the PTFE copolymerinclude, but are not limited to, olefins such as ethylene and propylene;halogenated olefins such as hexafluoropropylene (HFP), vinylidenefluoride (VDF), and chlorofluoroethylene (CFE); perfluoroalkyl vinylether (PPVE), and perfluorosulfonyl vinyl ether (PSVE). In yet anotherembodiment, the first and/or second PTFE membrane may be formed from ablend of high molecular weight PTFE homopolymer and a lower molecularweight modified PTFE polymer.

The first and second PTFE starting materials are selected so that, whentaken in combination with the selected processing conditions, theresultant first PTFE membrane has a lower matrix tensile strength thanthe second PTFE membrane. In at least one exemplary embodiment, thesecond PTFE membrane has a matrix tensile strength that is at least 1.5times greater than the first PTFE membrane in both the x- and they-directions.

A first PTFE membrane may be formed by blending a suitable first PTFEstarting material with a lubricant. Non-limiting examples of lubricantsfor use herein include light mineral oil, aliphatic hydrocarbons,aromatic hydrocarbons, and halogenated hydrocarbons. The resultingmixture of PTFE resin and lubricant may be formed into a cylindricalpellet and extruded through a die at a reduction ratio from about 10:1to about 150:1, or from about 25:1 to about 90:1 to form a tape. Thetape may then be calendered between rolls to a desired thickness at acalendering ratio from about 1.1:1 to about 50:1, or from about 1.1:1 toabout 20:1 to form the first PTFE membrane.

In at least one embodiment, the first PTFE membrane is formed without adrying step and is layered with the second PTFE membrane in a wet state.It is, however, within the purview of the invention to dry either thetape (pre-calendering) or the first PTFE membrane (post calendering)prior to layering with the second PTFE membrane.

Please note that although reference is made herein with respect to firstand second PTFE membranes for ease of discussion, a greater number ofPTFE membranes may be included in the composite article. In addition,the PTFE membranes within the composite article may be derived from thesame PTFE starting material, from a different PTFE starting material, ora combination of PTFE starting materials. Also, some or all of the PTFEmembranes in the composite article may vary in composition, bubblepoint, thickness, air permeability, mass/area, etc. from each other.

The second PTFE membrane may be formed by blending a second suitablePTFE starting material with a lubricant. Non-limiting examples oflubricants for use herein include light mineral oil, aliphatichydrocarbons, aromatic hydrocarbons, and halogenated hydrocarbons. Theresulting mixture may be formed into a cylindrical pellet and ramextruded through a die at a reduction ratio from about 10:1 to about150:1, or from about 50:1 to about 120:1 to form a tape. The tape maythen be calendered between rolls to a desired thickness at a calenderingratio from about 1.1 to about 20:1 or from about 1.1: to about 10:1. Thecalendered tape may then be expanded in one or more directions and driedto remove the lubricant. For example, the calendered tape may beexpanded in a longitudinal and/or a transverse direction at an expansionratio from about 1.1:1 to about 20:1 or about 1.1:1 to about 6:1. Theresultant second PTFE membrane has a node and fibril microstructure. Itis to be appreciated that the second PTFE membrane may be formed withoutdrying the tape and/or membrane and may be layered with the first PTFEmembrane in a wet state.

In forming the composite article, the first PTFE membrane and the secondPTFE membrane are layered or positioned one on top of the other in astacked configuration to form a layered product. The first and secondPTFE membranes may be positioned in a stacked configuration, forexample, by simply laying the membranes on top of each other.Embodiments employing two PTFE starting materials that are co-extrudedto produce a layered product is also considered to be within the purviewof the invention. The layered product may then be reduced to a desiredthickness, such as by calendering or placing the layered product in apress. The thickness of the layered product may range from about 0.01 mmto about 3.0 mm, from about 0.01 mm to about 2 mm, from about 0.03 mm toabout 1.0 mm, from about 0.05 mm to about 0.7 mm, or from about 0.1 mmto about 0.5 mm. Optionally, the layered product may be heated to removeany lubricant present. The layered product is biaxially stretched andoptionally sintered at a temperature above the crystalline melttemperature of the PTFE to form the composite article.

The layered product may be stretched in the x- and y-direction, eithersequentially or simultaneously. For instance, the layered product may bestretched in the y-direction at an average engineering strain rate fromabout 0.5%/sec to about 300%/sec, or from about 0.5%/sec to about150%/sec and a stretch amount from about 10% to about 350% or from about10% to about 300% and subsequently in the x-direction at an averageengineering strain rate from about 3% to about 600%, or from about 10%to about 400% and a stretch amount from about zero % to about 2000% orfrom about 1.0% to about 1600%, or vice versa (e.g., stretched first inthe x-direction and then stretched in the y direction). In at least oneembodiment, the layered product is simultaneously stretched in the x-and y-directions at an average engineering strain rate from about10%/sec to about 500%/sec, or from about 20%/sec to about 250%/sec and astretch amount from about 10% to about 2000%.

The resulting composite article has a unique macro textured surfacecharacterized by strands formed of either interconnected nodes of PTFEor of at least one nodal mass of PTFE. The strands have a length equalto or greater than about 1.5 mm and are raised from the surface of themembrane. Additionally, the strands may be visually observed and areeasily identified in an optical microscopic image. Looking at FIGS. 7and 8 as one exemplary embodiment, the strands can be easily seen atboth 7× and 32× magnification of the surface of the first PTFE membrane.The interconnected nodes may be individually observed with the use of ascanning electron microscope. A scanning electron micrograph of thesurface of the first PTFE membrane shown in FIG. 7 taken at 100×magnification clearly shows the individual nodes forming the strand.Each of the nodes is a solid or substantially solid mass of PTFE and arearranged or grouped together during the formation and subsequentexpansion of the first PTFE membrane so as to form one or more strandson the surface of the membrane. It is to be appreciated that remnants ofnodes may be present between the individual nodes.

In another exemplary embodiment, such as is shown in FIG. 13, thestrands are also visually observable on the surface of the first PTFEmembrane and are identifiable with an optical microscopic image. Thestrands are also raised from the surface of the first PTFE membrane.However, individual nodes are generally non-existent and one or morenodal mass of PTFE forms the strands.

At least one of the strands of interconnecting nodes and/or the strandscontaining one or more nodal mass of PTFE have a length equal to orgreater than about 1.5 mm, 2.0 mm, 3.0 mm, or 4.0 mm when measured atany two points along the strand when viewed optically, such as with anoptical microscope (e.g. at 7× or 32× magnification). In exemplaryembodiments, some or all of the strands have a length equal to orgreater than about 1.5 mm, 2.0 mm, 3.0 mm, or 4.0 mm when measured atany two points along the strand when viewed optically, such as with anoptical microscope (e.g. at 7× or 32× magnification). The strands form avisible pattern on the first PTFE membrane, and may be viewed with thenaked eye and/or an optical microscope. The strands of may run in agenerally parallel configuration to each other in the first PTFEmembrane in the y-direction. Additionally, the strands are generallynon-linear, and may run in a substantially curvilinear fashion. Inaddition, the individual strands may cross over or intersect each otherat one or more points on a strand or strands.

The composite article has a bubble point from about 3.0 psi to about 200psi, from about 5 psi to about 100 psi, from about 10 psi to about 85psi, or from about 15 psi to about 80 psi; a thickness from about 0.01mm to about 3.0 mm, from about 0.3 mm to about 1.5 mm, or from about0.05 mm to about 0.7 mm; and a bulk density from about 0.01 g/cm³ toabout 1.0 g/cm³, from about 0.01 g/cm³ to about 0.5 g/cm³, or from about0.05 g/cm³ to about 0.25 g/cm³. Additionally, the composite article mayhave a mass/area up to about 3000 g/m². In exemplary embodiments, thecomposite article has a mass/area from about 0.5 g/m² to about 750 g/m²,from about 0.5 g/m² to about 450 g/m², from about 5.0 g/m² to about 150g/m², or from about 10 g/m² to about 100 g/m²; and a Gurley number fromabout 0.01 second to about 1000 seconds or from about 1 second to about100 seconds. In one or more embodiment, the composite article is asheet, tape, or tube. The macro textured surface provides a topographyto the first PTFE membrane.

Optional support layers may be located between the first and second PTFEmembranes or adjacent to the first and/or second PTFE membrane.Non-limiting examples of suitable support layers include polymeric wovenmaterials, non-woven materials, knits, nets, and/or porous membranes. Inaddition, the first and second PTFE membranes may be adhered to eachother, or to another membrane or support structure, with a thermoplasticresin or other adhesive, either continuously or discontinuously, suchas, for example, fluorinated ethylene propylene (FEP), perfluoroalkoxypolymer resin (PFA), and tetrafluoroethylene hexafluoropropylene andvinylidene fluoride (THV), and/or polyvinylidene fluoride (PVDF).

The invention of this application has been described above bothgenerically and with regard to specific embodiments. It will be apparentto those skilled in the art that various modifications and variationscan be made in the embodiments without departing from the scope of thedisclosure. Thus, it is intended that the embodiments cover themodifications and variations of this invention provided they come withinthe scope of the appended claims and their equivalents.

Test Methods

It should be understood that although certain methods and equipment aredescribed below, other methods or equipment determined suitable by oneof ordinary skill in the art may be alternatively utilized.

Thickness

Membrane thickness was determined by placing the membrane between thetwo plates of a Mitutoyo Tektronix snap gauge (Part Number 547-400S).

Mass Per Area (Mass/Area)

The mass/area of the membrane was calculated by measuring the mass of awell-defined area of the sample using a scale. The sample was cut to adefined area using a die or any precise cutting instrument.

Density

The density was calculated by dividing the Mass per Area by Thickness.

Gurley Air Flow

The Gurley air flow test measures the time in seconds for 100 cm³ of airto flow through a 6.45 cm² sample at 12.4 cm of water pressure. Thesamples were measured in a Gurley Densometer Model 4340 AutomaticDensometer.

Bubble Point

The bubble point was measured according to the general teachings of ASTMF316-03 using a Capillary Flow Porometer (Model 3Gzh from QuantachromeInstruments). The sample holder comprised a porous metal plate (PartNumber: 04150-10030-25, Quantachrome Instruments), 25.4 mm in diameterand a plastic mask (Part Number ABF-300, Professional Plastics), 20mmI.D.×24.5 mm O.D. in diameter. The sample was placed in between themetal plate and the plastic mask. The sample was then clamped down andsealed using an o-ring (Part Number: 51000-25002000, QuantachromeInstruments). The sample was wet with the test fluid (Silicone fluidhaving a surface tension of 20.1 dynes/cm). Using the 3GWin softwareversion 2.1, the following parameters were set as specified in Table Iand Table II.

TABLE 1 Pore Pore Pore Pore Size Start Size End Size Start Size End RunSetting Pressure Pressure Size Size BP range (psi) (psi) (micron)(micron) BP_9-50 psi  8.97  50.48 1.3  0.231  BP_50-150 psi 50.7 149.10.23 0.0782 BP_50-120 psi 50.7 120   0.23 0.0972

TABLE 2 Parameter Bubble Point Run Type Wet Only Number Data Points 256Pressure Control Use Normal Equilibrium TRUE Use Tol FALSE Use TimeFALSE Use Rate FALSE Use Low Flow Sensor FALSE Time Out NA Equil Time NARun Rate NA Pressure Tolerance NA Flow Tolerance NA Smoothing UseMovAveFALSE MovAveWet Interval NA MovAveDry Interval NA Lowess Dry 0.050Lowess Wet 0.050 Lowess Flow 0.050 Lowess Num 0.100 MinSizeThreshold0.98 Bubble Point Parameters UseBpAuto TRUE UseBpThreshold (L/min) FALSEUseBpThreshold (Abs/cm²) FALSE UseBpThresholdNumber FALSEBpAutoTolerance (manual) 1% BpThresholdValue (manual) NA BpThreshold(Abs/cm²) 0 value

Optical Microscope Images

Optical microscopic images were generated using the Olympus SZX12microscope at a magnification of 7× and 32×.

SEM Sample Preparation Method

SEM images were generated using a high resolution field emissioncryogenic microscope (Hitachi S4700 FE-SEM).

EXAMPLES Example 1

A first polytetrafluoroethylene (PTFE) membrane was prepared as follows.A blend of a high molecular weight polytetrafluoroethylene fine powderand a lower molecular weight modified polytetrafluoroethylene polymerwas prepared in accordance with the teachings of U.S. Pat. No. 5,814,405to Branca, et al. and then combined with 0.244 lb/lb of lubricant(Isopar™ K, Exxon, Houston, Tex.). The resultant mixture was thenblended, compressed into a cylindrical pellet, and thermally conditionedfor 18 hours at a temperature of 25° C. The cylindrical pellet was thenextruded through a rectangular orifice die at a reduction ratio of 47:1to form a tape. The tape was then calendered between rolls at acalendering ratio of 14.39:1 to form the first PTFE membrane.

A second PTFE membrane was prepared as follows: A fine powder ofpolytetrafluoroethylene polymer made in accordance with the teachings ofU.S. Pat. No. 4,576,869 to Malhotra, et al. was combined with 0.21 lb/lbof lubricant (Isopar™ K, Exxon, Houston, Tex.). The resultant mixturewas then blended, compressed into a cylindrical pellet, and thermallyconditioned for 8 hours at a temperature of 49° C. The cylindricalpellet was then extruded through a rectangular orifice die at areduction ratio of 81:1 to form a tape. The tape was then calenderedbetween rolls at a calendering ratio of 3:1. The calendered tape wasthen transversely stretched at a ratio of 3.6:1 and dried at atemperature of 220° C. The width of the resulting second PTFE membranewas trimmed to match the width of the first PTFE membrane to facilitatethe layering process described below.

The first PTFE membrane was layered on the second PTFE membrane and thelayered product was calendered between rolls and reduced in thickness by36.4%. The resultant layered laminate was dried at a temperature of 250°C. to remove the lubricant. The dried laminate was then expanded at 300°C. in the y-direction at an average engineering strain rate of2.26%/second and a stretch amount equal to 60%. The resulting materialwas subsequently expanded in the x-direction at an average engineeringstrain rate of 19%/second, at a temperature of about 330° C. and astretch amount equal to 761° A. The material was then sintered at 360°C. for not more than 60 seconds.

The resultant composite article had a first surface and a secondsurface, representative of the first and second PTFE membranes,respectively, as described above. An optical microscopic image of thefirst surface of the composite article taken at 7× is shown in FIG. 1.An optical microscopic image of the first surface of the compositearticle taken at 32× is shown in FIG. 2. FIGS. 1 and 2 show the strandsof interconnected nodes forming a visible pattern on the first surface(i.e., the first PTFE membrane) of the composite article. FIG. 2A is ascanning electron micrograph (SEM) of the surface of the first PTFEmembrane in the composite article taken at 100× which shows theinterconnected nodes forming the strands.

The composite article had a bulk density of 0.24 g/cc. The bubble pointwas measured using the run setting BP_9-50psi. Two test conditions wererun. In the first test, the PTFE membrane of the composite article wasfacing the metal plate. In the second test, the second PTFE membrane ofthe composite article facing the metal plate. The bubble point wasmeasured to be 19.89 psi and 18.91, respectively.

Example 2

A first polytetrafluoroethylene (PTFE) membrane was prepared as follows.A blend of a high molecular weight polytetrafluoroethylene fine powderand a lower molecular weight modified polytetrafluoroethylene polymerwas prepared in accordance with the teachings of U.S. Pat. No. 5,814,405to Branca, et al. and then combined with 0.244 lb/lb of lubricant(Isopar™ K, Exxon, Houston, Tex.). The resultant mixture was thenblended, compressed into a cylindrical pellet, and thermally conditionedfor 18 hours at a temperature of 25° C. The cylindrical pellet was thenextruded through a rectangular orifice die at a reduction ratio of 47:1to form a tape. The tape was then calendered between rolls at acalendering ratio of 2.97:1 to form the first PTFE membrane.

A second PTFE membrane was prepared as follows. A fine powder ofpolytetrafluoroethylene polymer made in accordance with the teachings ofU.S. Pat. No. 4,576,869 to Malhotra, et al. was combined with 0.21 lb/lbof lubricant (Isopar™ K, Exxon, Houston, Tex.) The resultant mixture wasthen blended, compressed into a cylindrical pellet, and thermallyconditioned for 8 hours at a temperature of 49° C. The cylindricalpellet was then extruded through a die at a reduction ratio of 84:1 toform a tape. The tape was then calendered between rolls at a calenderingratio of 3:1. The calendered tape was then transversely stretched at aratio of 3.6:1 and dried at a temperature of 220° C. The width of theresulting second PTFE membrane was trimmed to match the width of thefirst PTFE membrane to facilitate the layering process described below.

The first PTFE membrane was layered on the second PTFE membrane and thelayered product was calendered between rolls and reduced in thickness by3.5%. The resultant layered laminate was dried at a temperature of 250°C. to remove the lubricant. The dried laminate was then expanded at 300°C. in the y-direction at an average engineering strain rate of2.26%/second and a stretch amount equal to 60%. The resulting materialwas subsequently expanded in the x-direction at an average engineeringstrain rate of 19.33%/second, at a temperature of about 300° C., and astretch amount equal to 761%. The material was then sintered at 390° C.for not more than 60 seconds.

The resultant composite article had a first surface and a secondsurface, representative of the first and second PTFE membranes,respectively, as described above. An optical microscopic image of thefirst surface of the composite article taken at 7× is shown in FIG. 3.An optical microscopic image of the first surface of the compositearticle taken at 32× is shown in FIG. 4. FIGS. 3 and 4 show the strandsof interconnected nodes forming a visible pattern on the first surface(i.e., the first PTFE membrane) of the composite article. FIG. 4A is ascanning electron micrograph (SEM) of the surface of the first PTFEmembrane in the composite article taken at 100× which shows theinterconnected nodes forming the strands.

The composite article had a bulk density of 0.14 g/cc. The bubble pointwas measured using the run setting: BP_9-50 psi. Two test conditionswere run. In the first test, the first PTFE membrane of the compositearticle was facing the metal plate. In the second test, the second PTFEmembrane of the composite article was facing the metal plate. The bubblepoint was measured to be 21.03 psi and 23.97 psi, respectively.

Example 3

A first polytetrafluoroethylene (PTFE) membrane was prepared as follows.A blend of a high molecular weight polytetrafluoroethylene fine powderand a lower molecular weight modified polytetrafluoroethylene polymerwas prepared in accordance with the teachings of U.S. Pat. No. 5,814,405to Branca, et al. and then combined with 0.244 lb/lb of lubricant(Isopar™ K, Exxon, Houston, Tex.). The resultant mixture was thenblended, compressed into a cylindrical pellet, and thermally conditionedfor 18 hours at a temperature of 25° C. The cylindrical pellet was thenextruded through a rectangular orifice to form a tape. The tape was thencalendered between rolls at a calendering ratio of 3.74:1 to form thefirst PTFE membrane.

A second PTFE membrane was prepared as follows. A fine powder ofpolytetrafluoroethylene polymer made in accordance with the teachings ofU.S. Pat. No. 4,576,869 to Malhotra, et al. was combined with 0.21 lb/lbof lubricant (Isopar™ K, Exxon, Houston, Tex.). The resultant mixturewas then blended, compressed into a cylindrical pellet, and thermallyconditioned for 8 hours at a temperature of 49° C. The cylindricalpellet was then extruded through a die at a reduction ratio of 84:1 toform a tape. The tape was then calendered between rolls at a calenderingratio of 3:1. The calendered tape was then transversely stretched at aratio of 3.6:1 and dried at a temperature of 220° C. The width of theresulting second PTFE membrane was trimmed to match the width of thefirst PTFE membrane to facilitate the layering process described below.

The first PTFE membrane was layered on the second PTFE membrane and thelayered product was reduced in thickness by 20.1%. The resultant layeredlaminate was then dried at a temperature of 250° C. to remove thelubricant. The dried laminate was then expanded at 300° C. in they-direction at an average engineering strain rate of 3.33%/sec and astretch amount equal to 85%. The resulting material was subsequentlyexpanded in the x-direction at an average engineering strain rate of19.33%/second, at temperature of about 300° C. and a stretch amountequal to 761%. The material was then sintered at 390° C. for not morethan 60 seconds.

The resultant composite article had a first surface and a secondsurface, representative of the first and second PTFE membranes,respectively, as described above. An optical microscopic image of thefirst surface of the composite article taken at 7× is shown in FIG. 5.An optical microscopic image of the first surface of the compositearticle taken at 32× is shown in FIG. 6. FIGS. 5 and 6 show the strandsof interconnected nodes forming a visible pattern on the first surface(i.e., the first PTFE membrane) of the composite article. FIG. 6A is ascanning electron micrograph (SEM) of the surface of the first PTFEmembrane in the composite article taken at 100× which shows theinterconnected nodes forming the strands.

The composite article had a bulk density of 0.15 g/cc. The bubble pointwas measured using the run setting BP_9-50 psi. Two test conditions wererun. In the first test, the first PTFE membrane the composite articlewas facing the metal plate. In the second test, the second PTFE membraneof the composite article was facing the metal plate. The bubble pointwas measured to be 35.05 psi and 45.16, respectively.

Example 4

A first polytetrafluoroethylene (PTFE) membrane was prepared as follows.A blend of a high molecular weight polytetrafluoroethylene fine powderand a lower molecular weight modified polytetrafluoroethylene polymerwas prepared in accordance with the teachings of U.S. Pat. No. 5,814,405to Branca, et al. and then combined with 0.259 lb/lb of lubricant(Isopar™ K, Exxon, Houston, Tex.). The resultant mixture was thenblended, compressed into a cylindrical pellet, and thermally conditionedfor 18 hours at a temperature of 25° C. The cylindrical pellet was thenextruded through a rectangular orifice die at a reduction ratio of 47:1to form a tape. The tape was then calendered between rolls at acalendering ratio of 15.9:1 to form the first PTFE membrane.

A second PTFE membrane was prepared as follows. A fine powder ofpolytetrafluoroethylene polymer made in accordance with the teachings ofU.S. Pat. No. 6,541,589 to Baillie was combined with 0.185 lb/lb oflubricant (Isopar™ K, Exxon, Houston, Tex.). The resultant mixture wasthen blended, compressed into a cylindrical pellet, and thermallyconditioned for 8 hours at a temperature of 49° C. The cylindricalpellet was then extruded through a rectangular orifice die at areduction ratio of 87:1 to form a tape. The tape was then calenderedbetween rolls at a calendering ratio of 3.9:1. The calendered tape wasthen transversely stretched at a ratio of 2.8:1 and dried at atemperature of 210° C. The width of the resulting second PTFE membranewas trimmed to match the width of the first PTFE membrane to facilitatethe layering process described below.

The first PTFE membrane was layered on the second PTFE membrane and thelayered product was reduced in thickness by 33.3%. The resultant layeredlaminate was then dried at a temperature of 210° C. to remove thelubricant. The dried laminate was then expanded at 300° C. in they-direction between heated drums at an average engineering strain rateof 3.7%/sec and a stretch amount equal to 300%. The resulting materialwas subsequently expanded in the x-direction at an average engineeringstrain rate of 3.66%/second, at temperature of about 300° C., and astretch amount equal to 162%. The material was then sintered at 390° C.for not more than 60 seconds.

The resultant composite article had a first surface and a secondsurface, representative of the first and second PTFE membranes,respectively, as described above. An optical microscopic image of thefirst surface of the composite article taken at 7× is shown in FIG. 7.An optical microscopic image of the first surface of the compositearticle taken at 32× is shown in FIG. 8. FIGS. 7 and 8 show the strandsof interconnected nodes forming a visible pattern on the first surface(i.e., the first PTFE membrane) of the composite article. FIG. 8A is ascanning electron micrograph (SEM) of the surface of the first PTFEmembrane in the composite article taken at 100× which shows theinterconnected nodes forming the strands.

The composite article had a bulk density of 0.43 g/cc. The bubble pointwas measured using the run setting BP_50-150 psi. Two test conditionswere run. In the first test, the first PTFE membrane of the compositearticle was facing the metal plate. In the second test, the second PTFEmembrane of the composite article was facing the metal plate. The bubblepoint was measured to be 78.10 psi and 81.58 respectively.

Example 5

A first polytetrafluoroethylene (PTFE) membrane was prepared as follows.A blend of a high molecular weight polytetrafluoroethylene fine powderand a lower molecular weight modified polytetrafluoroethylene polymerprepared in accordance with the teachings of U.S. Pat. No. 5,814,405 toBranca, et al. was combined with 0.259 lb/lb of lubricant (Isopar™ K,Exxon, Houston, Tex.). The resulting mixture was then blended,compressed into a cylindrical pellet, and thermally conditioned for 18hours at a temperature of 25° C. The cylindrical pellet was thenextruded through a rectangular orifice die at a reduction ratio of 47:1to form a tape. The tape was then calendered between rolls at acalendering ratio of 4.97:1 to form the first PTFE membrane.

A second PTFE membrane was prepared as follows. A fine powder ofpolytetrafluoroethylene polymer made in accordance with the teachings ofU.S. Pat. No. 6,541,589 to Baillie was combined with 0.185 lb/lb oflubricant (Isopar™ K, Exxon, Houston, Tex.). The resulting mixture wasthen blended, compressed into a cylindrical pellet, and thermallyconditioned for 8 hours at a temperature of 49° C. The cylindricalpellet was then extruded through a rectangular orifice die at areduction ratio of 87:1 to form a tape. The tape was then calenderedbetween rolls at a calendering ratio of 3.9:1. The calendered tape wasthen transversely stretched at a ratio of 3.38:1 and dried at atemperature of 180° C. and 210° C. in a first and second oven,respectively, to form the second PTFE membrane.

The first PTFE membrane was layered on the second PTFE membrane and thelayered product was reduced in thickness by 41.6%. The resultant layeredlaminate was then dried at a temperature of 210° C. to remove thelubricant. The dried laminate was then expanded at 300° C. in they-direction between heated drums at an average engineering strain rateof 0.9%/sec and a stretch amount equal to 50%. The resulting materialwas subsequently expanded in the x-direction at an average engineeringstrain rate of 61%/second, at temperature of about 300° C., and astretch amount equal to 1565%. The material was then sintered at 380° C.for not more than 60 seconds. .

The resultant composite article had a first surface and a secondsurface, representative of the first and second PTFE membranes,respectively, as described above. An optical microscopic image of thefirst surface of the composite article taken at 7× is shown in FIG. 9.An optical microscopic image of the first surface of the compositearticle taken at 32× is shown in FIG. 10. FIGS. 9 and 10 show thestrands of interconnected nodes forming a visible pattern on the firstsurface (i.e., the first PTFE membrane) of the composite article. FIG.10A is a scanning electron micrograph (SEM) of the surface of the firstPTFE membrane in the composite article taken at 100× which shows theinterconnected nodes forming the strands.

The composite article had a bulk density of 0.09 g/cc. The bubble pointwas measured using the run setting BP_50-120 psi. Two test conditionswere run. In the first test, the first PTFE membrane of the compositearticle was facing the metal plate. In the second test, the second PTFEmembrane of the composite article was facing the metal plate. The bubblepoint was measured to be 75.8 psi and 81.93 psi respectively.

Example 6

6 A first polytetrafluoroethylene (PTFE) membrane was prepared asfollows. A fine powder of polytetrafluoroethylene polymer (Part Number:T62, DuPont., Parkersbury, W.Va.) was combined with 0.235 lb/lb oflubricant (Isopar™ K, Exxon, Houston, Tex.). The resultant mixture wasthen blended, compressed into a cylindrical pellet, and thermallyconditioned for 8 hours at a temperature of 16° C. The cylindricalpellet was then extruded through a rectangular orifice die at areduction ratio of 74:1 to form a tape. The tape was then calenderedbetween rolls at a calendering ratio of 1.65:1 to form the first PTFEmembrane.

A second PTFE membrane was prepared as follows. A fine powder ofpolytetrafluoroethylene polymer made in accordance with the teachings ofU.S. Pat. No. 4,576,869 to Malhotra, et al. was combined with 0.226lb/lb of lubricant (Isopar™ K, Exxon, Houston, Tex.). The resultantmixture was then blended, compressed into a cylindrical pellet, andthermally conditioned for 8 hours at a temperature of 49° C. Thecylindrical pellet was then extruded through a die at a reduction ratioof 74:1 to form a tape. The tape was then calendered between rolls at acalendering ratio of 3.7:1 to form the second PTFE membrane.

The first PTFE membrane was layered on the second PTFE membrane and thelayered product was reduced in thickness by 19%. The layered laminatewas transversely stretched at a ratio of 3.6:1 and subsequently dried ata temperature of 250° C. to remove the lubricant. The dried laminate wasthen expanded at 300° C. in the y-direction between heated drums at anaverage engineering strain rate of 109%/sec and a stretch amount equalto 25%. The resulting material was subsequently expanded in thex-direction at an average engineering strain rate of 399%/second, atemperature of about 260° C., and a stretch amount equal to 467%. Thematerial was then sintered at 373° C. for not more than 60 seconds.

The resultant composite article had a first surface and a secondsurface, representative of the first and second PTFE membranes,respectively, as described above. An optical microscopic image of thefirst surface of the composite article taken at 7× is shown in FIG. 11.An optical microscopic image of the first surface of the compositearticle taken at 32× is shown in FIG. 12. FIGS. 11 and 12 show thestrands of interconnected nodes forming a visible pattern on the firstsurface (i.e., the first PTFE membrane) of the composite article. FIG.12A is a scanning electron micrograph (SEM) of the surface of the firstPTFE membrane in the composite article taken at 100× which shows theinterconnected nodes forming the strands.

The composite article had a thickness of 0.139 mm and a bulk density of0.25 g/cc. The bubble point was measured using the run setting: BP_9-50psi. Two test conditions were run. In the first test, the first PTFEmembrane of the composite article was facing the metal plate. In thesecond test, the second PTFE membrane of the composite article wasfacing the metal plate. The bubble point was measured to be 23.64 psiand 24.29 psi, respectively.

Example 7

A first polytetrafluoroethylene (PTFE) membrane was prepared as follows.A fine powder of polytetrafluoroethylene polymer (Part Number: T62,DuPont., Parkersbury, W.Va.) was combined with 0.253 lb/lb of lubricant(Isopar™ K, Exxon, Houston, Tex.). The resultant mixture was thenblended, compressed into a cylindrical pellet, and thermally conditionedfor 8 hours at a temperature of 16° C. The cylindrical pellet was thenextruded through a rectangular orifice die at a reduction ratio of 74:1to form a tape. The tape was then calendered between rolls at acalendering ratio of 1.65:1 to form the first PTFE membrane.

A second PTFE membrane was prepared as follows. A fine powder ofpolytetrafluoroethylene polymer made in accordance with the teachings ofU.S. Pat. No. 4,576,869 to Malhotra, et al. was combined with 0.226lb/lb of lubricant (Isopar™ K, Exxon, Houston, Tex.). The resultantmixture was then blended, compressed into a cylindrical pellet, andthermally conditioned for 8 hours at a temperature of 49° C. Thecylindrical pellet was then extruded through a rectangular orifice dieat a reduction ratio of 74:1 to form a tape. The tape was thencalendered between rolls at a calendering ratio of 3.7:1 to form thesecond PTFE membrane.

The first PTFE membrane was layered on the second PTFE membrane and thelayered product was reduced in thickness by 50.8%. The layered laminatewas transversely stretched at a ratio of 3.6:1 and then dried at atemperature of 250° C. to remove the lubricant. The dried laminate wasthen expanded at 300° C. in the y-direction between heated drums at anaverage engineering strain rate of 14%/sec and a stretch amount equal to103%. The resulting material was subsequently expanded in thex-direction at an engineering strain rate of 273%/second, a temperatureof 260° C., and a stretch amount equal to 467%. The material was thensintered at 373° C. for not more than 60 seconds.

The resultant composite article had a first surface and a secondsurface, representative of the first and second PTFE membranes,respectively, as described above. An optical microscopic image of thefirst surface of the composite article taken at 7× is shown in FIG. 13.An optical microscopic image of the first surface of the compositearticle taken at 32× is shown in FIG. 14. FIGS. 13 and 14 show thestrands of interconnected nodes forming a visible pattern on the firstsurface (i.e., the first PTFE membrane) of the composite article. FIG.14A is a scanning electron micrograph (SEM) of the surface of the firstPTFE membrane in the composite article taken at 100× which shows theinterconnected nodes forming the strands.

The composite article had a thickness of 0.134 mm and a bulk density of0.17 g/cc. The bubble point was measured using the run setting BP_9-50psi. Two test conditions were run. In the first test, the first PTFEmembrane of the composite article was facing the metal plate. In thesecond test, the second PTFE membrane of the composite article wasfacing the metal plate. The bubble point was measured to be 21.03 psiand 20.87 psi, respectively.

Example 8

A first polytetrafluoroethylene (PTFE) membrane was prepared as follows.A polytetrafluoroethylene polymer fine powder resin (Part Number: F104,Daikin., Ala.) was combined with 0.252 lb/lb of lubricant (Isopar™ K,Exxon, Houston, Tex.). The resultant mixture was then blended,compressed into a cylindrical pellet, and thermally conditioned for 8hours at a temperature of 23° C. The cylindrical pellet was thenextruded through a rectangular orifice die at a reduction ratio of 40:1to form a tape. The tape was then calendered between rolls at acalendering ratio of 2.75:1 to form the first PTFE membrane.

A second PTFE membrane was prepared as follows. Apolytetrafluoroethylene polymer fine powder made in accordance with theteachings of U.S. Pat. No. 4,576,869 to Malhotra, et al. was combinedwith 0.186 lb/lb of lubricant (Isopar™ K, Exxon, Houston, Tex.). Theresultant mixture was then blended, compressed into a cylindricalpellet, and thermally conditioned for 8 hours at a temperature of 70° C.The cylindrical pellet was then extruded through a rectangular orificedie at a reduction ratio of 78:1 to form a tape. The tape was thencalendered between rolls at a calendering ratio of 3.4:1 to form thesecond PTFE membrane.

The first PTFE membrane was layered on the second PTFE membrane and thelayered product calendered between rolls. The resultant layered laminatewas then dried at a temperature of 250° C. to remove the lubricant. Thedried laminate was then expanded at 300° C. in the y-direction betweenheated drums at an average engineering strain rate of 99%/sec and astretch amount equal to 30%. The resulting material was subsequentlyexpanded in the x-direction at an average engineering strain rate of228%/second, a temperature of about 300° C., and a stretch amount equalto 1600%. The material was then sintered at 380° C. for not more than 60seconds.

The resultant composite article had a first surface and a secondsurface, representative of the first and second PTFE membranes,respectively, as described above. An optical microscopic image of thefirst surface of the composite article taken at 7× is shown in FIG. 15.An optical microscopic image of the first surface of the compositearticle taken at 32× is shown in FIG. 16. FIGS. 15 and 16 show thestrands of interconnected nodes forming a visible pattern on the firstsurface (i.e., the first PTFE membrane) of the composite article. FIG.16A is a scanning electron micrograph (SEM) of the surface of the firstPTFE membrane in the composite article taken at 100× which shows theinterconnected nodes forming the strands.

The composite article had a thickness of 0.171 mm and a bulk density of0.16 g/cc. The bubble point was measured using the run setting BP_9-50psi. Two test conditions were run. In the first test, the first PTFEmembrane of the composite article was facing the metal plate. In thesecond test, the second PTFE membrane of the composite article wasfacing the metal plate. The bubble point was measured to be 23.64 psiand 26.25 psi, respectively.

The invention of this application has been described above bothgenerically and with regard to specific embodiments. It will be apparentto those skilled in the art that various modifications and variationscan be made in the embodiments without departing from the scope of thedisclosure. Thus, it is intended that the embodiments cover themodifications and variations of this invention provided they come withinthe scope of the appended claims and their equivalents.

1.-27. (canceled)
 28. A method of forming a composite article comprising: obtaining a first PTFE membrane having a first surface and a second surface; obtaining a second PTFE membrane having a matrix tensile strength that is greater than a matrix tensile strength of the first PTFE membrane; positioning said second PTFE membrane on said second surface of said first membrane to form a layered product; expanding said layered product in the y direction at an engineering strain rate from about 0.5%/sec to about 300%/sec and a stretch rate from about 10% to about 350% to form an expanded product; and expanding said expanded product in the x-direction at an engineering strain rate from about 3% to about 600% and a stretch ratio from about 0% to about 2000% to form a composite article, wherein said first surface of said composite article includes a plurality of strands raised from said first surface, each said strand having a length equal to or greater than about 1.5 mm, wherein said first surface of said composite article corresponds to said first surface of said first PTFE membrane.
 29. The method of claim 28, further comprising sintering said composite article.
 30. The method of claim 28, further comprising applying an adhesive to one of said first PTFE membrane and said second PTFE membrane to adhere said first PTFE membrane to said second PTFE membrane. 